Long distant transmission using optical fiber cables (hereinafter referred as optical cables, occasionally) is performed in order to transmit a large volume of data at a high speed, in recent years. In the long distance transmission, a transmission loss generates in an optical cable. For this, the optically amplified transmission system transmits transmission light and reception light thorugh different optical cables in order to compensate it.
The optically amplified transmission system can remotely control an amplification level of an optical amplifier in the optical repeater. When the optical cables are installed undersea, for example, it is possible to remotely adjust the amplification level using an optical cable instead the administrator adjusts the amplification level by hands. Concretely, this adjustment is carried out by inputting pumping light to the optical amplifier, which is called the optical pumping system. In the optical pumping system, the pumping light of a transmission path terminal station is transmitted to an optical repeater, the transmitted pumping light and transmission light are multiplexed in the optical repeater, whereby optical repeating is performed.
In order to perform the optical repeating, a different optical cable from optical cables through which the transmission light and the pumping light are transmitted is installed, and these optical cables are prepared for each repeater to transmit optical signals. These repeaters are connected by the optical cables.
When a fault occurs in the optical cable and the optical cable is cut, it is necessary to specify the cut position and restore it. Detection of cut of an optical cable is called disconnect detection of an optical cable. The disconnect detecting function is dispensable.
FIG. 47 is a diagram showing an example of the optically amplified transmission system using the remote pumping optically amplifying system. An optically amplified transmission system 90 shown in FIG. 47 is described in Japanese Patent Laid-Open Publication No. 9-113941 in which a technique is disclosed, which can further extend a transmission distance of optical signals using an optically amplifying system which can do remote pumping.
The optically amplified transmission system 90 shown in FIG. 47 comprises a transmitting station (transmitting terminal) 90a, a receiving station (receiving terminal) 90b, a plurality (three, for example) of repeating stations 90c, and a plurality (three, for example) of repeating stations 90d between them. Optical cables in two systems are used for transmission and reception.
The transmitting station 90a comprises a transmitting unit 91a and a receiving unit 91b. The receiving station 90b comprises a transmitting unit 91d and a receiving unit 91c, as well. The transmitting unit 91a and the receiving unit 91d comprise a signal light source 92a and a plurality of pumping sources 92b. Each of the transmitting station 90a and the receiving station 90b prepares the pumping sources 92b in different systems from that of the signal light source in order to perform remote pumping, whereby pumping light is transmitted from a terminal station to a repeater through an optical cable differing from an optical cable for transmission-reception light between the transmitting station 90a and the receiving station 90b. 
Japanese Patent Laid-Open Publication No. 9-200144 discloses an optically amplified repeating system which can suppress the output level of a repeater apparatus to realize a long repeater spacing. According to the technique disclosed in this publication, it is possible to extend the repeating distance.
However, the techniques disclosed in Japanese Patent Laid-Open Publication No. 9-113941 and Japanese Patent Laid-Open Publication No. 9-200144 have three types of problems. First, since a plurality of pumping sources are transmitted through optical cables differing from an optical cable through which optical signals are transmitted, the investment cost of the optical cables is high. In concrete, in the optically amplified transmission system 90 shown in FIG. 47, the number of the optical cables required among the optical repeaters is two to three, and the number of optical cables required between the terminal stations is 14 for both transmission and reception. Further, no description of the optical level controlling method is seen therein.
Further, in remote pumping, adjustment of the output level between the optical repeaters sometimes lacks accuracy since the adjustment is carried out on the basis of theoretical optical transmission distance calculation. The second problem is that a precise control is necessary in each terminal station, which requires a labor cost of an administrator who executes the control.
The third problem is that the method for detecting cut of an optical cable is not established.
In the light of the above problems, the first object of the present invention is to enable transmission and reception of transmission light and reception light through one optical fiber cable, thereby decreasing the installation cost and maintenance cost of the optical cables.
The second object of the present invention is to detect cut of the optical cable by a monitoring function using pumping light and residual pumping light, thereby largely improving reliability and safety of the system. The third object of the present invention is to most suitably set optical output level adjustment in the repeating station according to an actual transmitting distance.